The businesswoman at the centre of the legal challenge to ensure parliament is consulted before Theresa May triggers Brexit has said the landmark case was motivated by her fear that the UK faced a “treacherous future”.

In an interview with the Guardian, Gina Miller said she knew the ruling would leave her unpopular with many EU referendum voters, but believed that the UK had failed itself and the rest of Europe by voting to leave the bloc rather than reform it from within.

“I was never binary remain or leave. I was very much of the sentiment, and still am, that it was about remain, reform and review,” Miller said. “The UK actually has a very powerful place in Europe ... and we have not just let ourselves down but I think the whole of Europe down by not taking up that challenge.”

Before launching her case, the 51-year-old who runs the investment firm SCM Private with her husband, Alan, had spent a decade campaigning for transparency in investment and pension funds, and for reform in the charity sector. “I’ve stood up and made myself very unpopular,” she said. “But it’s not about being unpopular, it’s about doing the right thing.”

The anxiety that Miller felt after the referendum result was shared by many – but most people did not summon their lawyers and launch a high-profile court case. “It’s just in my nature, it’s what I do,” Miller said. “I tend to stand up and speak up when I see something dysfunctional happening.”

Her previous campaigns may have made her unpopular in certain circles – she told the Times last month she was known as the “black widow spider” by some in the City. But that is nothing compared to the vitriol and even death threats she has been exposed to since emerging as the principal legal challenger to the government’s Brexit plans. Following the court ruling, people took to social media describing her as a “foreign-born” “rich bitch” and asking who she thought she was.

She would not discuss the abuse she has received in detail. “It’s been considerable,” she said. “All I will say is that people who wanted to support me have done so quietly rather than audibly because they’re frightened.”

Miller is from Guyana but has lived in the UK since she was 10. Asked whether she believed a white man would have received similar levels of hate she laughed, not entirely mirthfully. “No,” she replied. “The simple answer: absolutely no.”

The reticence of others to publicly support her had been disheartening at times. “But overall it empowers me to think that this is exactly why I need to do this, because we have to come together,” she said. “We can’t have this fractured society that’s resulting at the moment.”

Miller has experienced both poverty and affluence: born in what was then British Guiana, she was sent to the UK by her parents to attend Roedean, the prestigious Sussex boarding school, but ran away after being bullied, she told the Times – an experience she said “made me steely very early”.

She has described herself as a “driven misfit” who at one stage aspired to be a barrister but was told “the criminal bar was not the place for a woman”.

She married young and had a disabled daughter, now in her late 20s, who still lives with her. She had two more children in her early 40s with Alan, her third husband. At various stages of her career she has been a chambermaid, a model and an entrepreneur. “I have always been drawn to male-dominated industries and adrenaline-filled pursuits,” she has written.

Facing scores of reporters and TV crews on the steps of the Royal Courts of Justice following the judgment, Miller was unruffled. “The result today is about all of us, it’s not about me or my team,” she said. “It’s about our UK and all our futures ... This case was about process, not politics.”

Miller denies her lawsuit is trying to reverse the referendum. “I don’t think that’s possible,” she told the Guardian. Instead, she says it is an attempt to gain legal clarity over whether the government is entitled to trigger article 50, or whether this must be approved by parliament.

“All the people who have been saying ‘we need to take back control’, ‘we need sovereignty’, well you can’t have it with one hand and then with the other say, ‘I’m going to bypass it now and not seek consultation from the representatives in parliament,’” she said. “You just can’t have it both ways.

But she added: “Unless we have the legal certainty to know what we’re doing as we go forward is binding, then who knows what the future will have in store for us? We could be walking into so many legal challenges and nightmares. Isn’t it better that the legal certainty is ruled [on] now rather than later?”

Miller also has philanthropic projects, including the True and Fair Campaign for transparency and scrutiny of pension funds.

She told the Guardian she viewed the legal action as a form of public service. “One of my doctrines, my central principles of my life has been what I call conscious capitalism ... I’ve worked for everything I’ve had, and I can’t think of a better way of using it than standing up for what’s right, and what’s required to build a better society. So for me it’s an extension very much of my philanthropic work.”

Miller is funding her case from her own pocket, although she will not discuss how much she has spent so far or how much she expects to spend. “It’s been way beyond just monetary,” she said. And now she faces a final push as the case heads to the supreme court on 5 December, leapfrogging the court of appeal.

Although jubilant about the high court’s ruling, she said she was disappointed at the speed with which the government announced it would appeal “rather than taking time to read the full judgement and consider their position”. She said: “I just really don’t see where they would get anything different in the supreme court from what they’ve got today.”

Miller was more certain than ever that her legal challenge was the right thing to do. “I’ve never walked away – and actually what has unfolded, the level of division has meant it’s made me even [more convinced] there was no alternative,” she said. “This had to be done.”